As is known, the heads of femoral stems of hip prostheses are generally articulated on a polyethylene component lodged in the metal socket, which itself is impacted in the osseous wall of the acetabular cavity. The polyethylene component forms a socket internal to the metal socket and fixed to the latter. Now, the frictions between the metal or ceramic head and the polyethylene insert component cause certain problems. In addition, these prostheses present risks of dislocation.
The problem caused by the appearance of polyethylene debris resulting from the friction of the head of the prosthesis against the polyethylene socket is well known. This wear leads to the formation of polyethylene micro-debris which itself provokes a macrophage reaction, which itself leads to the production of a membrane disposed between the implants and the bone, this membrane itself being a cause of separation.
The problem of hip dislocation is substantially different. It is known that the hip prostheses comprising a head of large diameter, that is to say of at least 32 mm, are unlikely to dislocate and are relatively stable; the prostheses with a greater diameter, for example of the MAC KEE, FARRART or THOMSON types, are even less dislocatable. Moreover, for some time now hip prosthesis heads have been used with a considerably smaller diameter, for example 28 mm, in order to reduce wear, since the coefficient of friction between such a head and the polyethylene is less than with a head which has a greater diameter.
However, this has not sufficed to eliminate wear and has led to another problem, namely that of the instability of the hips. The reason is that as the coefficient of friction is lower, the hip is more dislocatable, particularly in the period immediately following surgery.
Ceramic-ceramic friction pairings are also known and afford advantages. However, these pairings lead to other difficulties, especially risks of separation and dislocation.
The document EP-A-0 461 019 describes a hip prosthesis comprising a spherical head articulated in an elastic female part of a cup articulated in a socket. The head can leave the elastic female part, which renders the prosthesis dislocatable and thus constitutes a very considerable disadvantage.
Finally, prostheses are known which include a cup of large diameter which is impacted on a metal head with interposition of flexible polyethylene in order to permit the impaction and retention on the metal head. This system thus still entails metal-polyethylene wear. Moreover, this type of prosthesis causes cotyloid inflammation because they are arranged in osseous sockets, and one then observes an osseous reaction to the metal implant situated opposite.